1. Field of the Invention
This present invention is directed to directional drilling, systems for controlling the orientation of a drill bit during directional drilling, and, in certain particular aspects, to applying specific amounts of energy to a drillstring to prevent binding of the drillstring while maintaining bit face orientation during drilling.
2. Description of Related Art
The prior art discloses a wide variety of drilling systems, directional drilling systems, apparatuses, and methods; including, but not limited to, the disclosures in U.S. Pat. Nos. 6,944,547; 6,918,453; 6,802,378; 6,050,348; 5,465,799; 4,995,465; 4,854,397; and 3,658,138, all incorporated fully herein for all purposes.
In directional drilling target formations may be spaced laterally thousands of feet from a well's surface location requiring penetration to depth and also laterally through soil, rock, and formations.
In directional drilling a substantial length of the drillstring is in frictional contact with and supported by the borehole. Since the drillstring is not rotating, it is difficult to overcome the friction. The difficulty in overcoming the friction makes it difficult for the driller to apply sufficient weight to the bit to achieve an optimal rate of penetration. The drillstring exhibits stick/slip friction such that when a sufficient amount of weight is applied to overcome the friction, the drill the weight on bit tends to overshoot the optimum magnitude. Additionally, the reactive torque that would be transmitted from the bit to the surface through drillstring, if the hole were straight, is absorbed by the friction between the drillstring and the borehole. Thus, during drilling, there can be substantially no reactive torque at the surface. Moreover, in certain methods when the driller applies drillstring angle corrections at the surface in an attempt to correct the bit face angle, a substantial amount of the angular change is absorbed by friction without changing the face angle in stick/slip fashion. When enough angular correction is applied to overcome the friction, the face angle may overshoot its target, thereby requiring the driller to apply a reverse angular correction.
One of the challenges for directional drilling is preventing the horizontal portions of the string from binding with the surrounding rock formation and reducing sliding friction as the bit progresses through the formation. To do this it is desirable to keep as much of the horizontal portion of the string moving as fast as possible without affecting or changing the bit face orientation. The process of rotating the string alternating in direction without moving the bit is known as “wagging the dog.”
In one prior method the frictional engagement between the drillstring and the borehole is reduced by rocking the drillstring back and forth between a first angle and a second angle. By rocking the string, the stick/slip friction is reduced, thereby making it easier for the driller to control the weight on bit and make appropriate face angle corrections. In another prior method a motor's shaft is rotated at a fixed speed for a specified time in each direction, effectively rotating the shaft a fixed distance.
Another prior method and system for directional drilling reduces the friction between the drillstring and the well bore by rocking the drillstring back and forth between first and second torque magnitudes. A downhole drilling motor is connected to the surface by a drillstring. The drilling motor is oriented at a selected tool face angle. The drillstring is rotated at the surface location in a first direction until a first torque magnitude is reached, without changing the tool face angle. The drillstring is then rotated in the opposite direction until a second torque magnitude is reached, again without changing the tool face angle. The drillstring is rocked back and forth between the first and second torque magnitudes.
In rotating a drillstring to overcome undesirable sticking and friction, ideally such rotation does not change the bit face orientation; but, due to the limitations of available prior art systems and methods, it is often difficult to nullify frictional effects on the drillstring without adversely changing bit face orientation.